1. Field of the invention
This invention relates to an absorbent fibrous structure and producing method thereof. The absorbent fibrous structure is characterized by that it can keep its original shape after water is absorbed into it. Then, the inner surface of a food container is covered with the absorbent fibrous sheet in order to absorb juice, gravy, dew condensation and the like from the food. Also, the absorbent fibrous sheet can be used as a wrapping sheet with which vegetables and fruits are wrapped in order to keep their freshness by absorbing dew condensation from them. Further, this absorbent fibrous sheet can be used as a building sheet with which the ceiling and the wall of a building or the inner wall of a container is covered in order to absorb dew condensation from the building or the container. Alternatively, water is previously absorbed into absorbent fibrous sheet before food is wrapped with this sheet. In case that the food is wrapped with this sheet and heated in a micro-wave oven, vapor can be obtained from this absorbent fibrous sheet. In case that the food is fish or meat and is wrapped with this sheet so as to be kept, the freshness of the food can be maintained due to the water absorbed into the absorbent fibrous sheet. In the present invention, the absorbent fibrous structure is not limited to the sheet-structure and involves also other molded structures.
2. Description of the prior art
Fresh provision is packed in a film tray pack for commercial display. That is to say, the fresh provision is put on a tray made from polystyrene foam and covered with transparent film. However, since liquid such as blood and gravy from fish or meat deteriorates the appearance of the packed provision, some kinds of absorbent sheet capable of keeping their original shapes after absorbing the liquid are used to lay between the tray and the provision. Also in order to dehumidify boiled rice, the absorbent sheet is used. Further, in order to maintain freshness of some kinds of food, dew condensation from the food such as vegetables, fruits and the like should be absorbed into the absorbent sheet. Further, in order to absorb dew condensation generated in a room, a building, the absorbent sheet is used. In this case, this sheet is required to be suitable for interior work. One example of conventional absorbent sheets utilized in such situations is a water absorptive sheet, which is produced by coating water absorptive resin powders together with binder on a non-woven fabric or a plastic sheet. Another example of conventional absorbent sheets is a dry-laid non-woven fabric containing high absorptive fibers. However, these conventional absorbent sheets have disadvantages. For example; swelling gel particles are easily removed from the resin coated absorbent sheets. Then, since the binder obstructs absorbency, enough water cannot be absorbed into the water absorptive sheet. On the other hand, as for the dry-laid non-woven fabric, its utilization is limited due to its appearance; fabric.
Alternatively, several kinds of absorbent sheet for wrapping food have been disclosed. First, a laminated structure in the publication of Japanese Utility Model Application (Kokoku) No. 54-5349 is produced by laminating gas permeable-foamed substances comprising polyethylene with a sheet of cellulosic fibers. However, this laminated structure does not have high degree of water absorption and can not surely keep the absorbed water. Next, a laminated sheet in the publication of unexamined Japanese Utility Model Application (Kokai) No. 58-49779 is produced by laminating a water absorptive resin layer formed with cross-linked poly(sodium acrylate) and the like with gas and liquid permeable layers, such as papers, fabrics or porous sheets. The resin layer is placed on one side of the permeable layer or between the permeable layers. However, when this laminated sheet is produced, adhesive is required for keeping and joining its structure. Therefore, it is not so easy to produce this structure. Thirdly, in the publication of unexamined Japanese Utility Model Application (Kokai) No. 1-163123, a water absorptive sheet is disclosed. This sheet is produced by putting porous protective sheets on the both sides of a non-woven absorbent sheet made of a fibrous water absorptive polymer, such as acrylonitrile-acrylate copolymer. That is to say, this sheet must be provided with the protective sheets comprising thermofusible fibers on its both sides. Therefore, film made from thermoplastic resin or non-woven fabric formed with thermoplastic resin-fibers as the protective sheet should be laminated with adhesive. Alternatively, the film or the fabric should be heated so as to be sticked together.
First, the above mentioned conventional absorbent sheets do not have enough water absorbency. Secondly, the conventional absorbent sheets must be provided with extra surface layers. Because, the sheets often must be prevented from contacting food directly, the water absorptive resin in the absorbing layer and the absorbing non-woven fabrics must be covered and the surfaces of the conventional absorbent sheets must be clear so that slime is not generated. Thirdly, the conventional absorbent sheets can not be applied to other molded structures. That is to say, only sheet-structure is used.
A following phenomenon has been known well. Multifunctional chemicals are added to water-soluble salts of fibrous carboxymethyl cellulose (Hereinafter, fibrous carboxymethyl cellulose is sometimes expressed by "fibrous CMC" as desired) so that cross-linking can be formed. Thus, the salts become insoluble in water and swollen gel is generated. As shown in U.S. Pat. No. 3,589,364, this phenomenon is applied to bibulous cellulosic fibers. However, since an absorbent fibrous sheet produced from these fibers does not have enough strength, this sheet can not be used actually for the above utilization as it is. Alternatively, the cross-linking is formed by adding multifunctional resin such as polyamide-polyamine epichlorohydrin resin to the fibrous CMC. Then, a paper-forming process is carried out for the fibrous CMC having cross-linking followed by a treatment in an alkaline solution so that water-soluble salts of the fibrous CMC can be formed. Then, an absorbent fibrous sheet having higher wet-web strength can be produced. However, the surface of this absorbent fibrous sheet is alkalized. Accordingly, when the sheet is contacted to food or a human body directly, this sheet would be unsuitable as it is. Further, the absorbed water is mainly taken into the swollen gel of the water-soluble salts of the fibrous CMC. Therefore, slime is generated on the surface. Also from this point, the surface layers are necessary.